Modern telecommunications networks demand a high degree of synchronisation between network transmission elements. For all network transmission elements in SDH architectures, timing is critical. However, phase variations in the reference clock frequencies governing synchronous network elements may introduce errors at various stages in the network.
Degradation of synchronisation in an SDH network may be due to several factors. Common causes include variations in propagation times in cabling and frequency drifts due to temperature changes in the Phase Locked Loops (PLLs) used. Errors in synchronisation may also occur if a Synchronisation Supply Unit (SSU) or SSDH Equipment Clock (SEC) operates out of the ideal locked mode and in hold-over or free-running modes. Any general re-configuration event in the synchronisation chain may give rise to transient events, as will a change of a Primary Reference Clock (PRC) in international links.
Variations in the timing signal may be broadly split into two categories. In the ITU specifications, short term variations which are of frequency greater than or equal to 10 Hz are referred to as “jitter”. Longer term variations, which are of frequency less than 10 Hz, are referred to as “wander”.
Since there are strict regulations governing timing it is necessary to have some means of measuring and identifying faults and errors. Three important measurements of network timing errors in the ITU recommendations are the Time Interval Error (TIE), Maximum TIE (MTIE), and Time Deviation (TDEV). Of principal interest for the present invention is TDEV, which is a measure of the time variation of a signal over a specific integration time (observation interval). TDEV is measured in units of time and is derived from a sequence of Time Interval Error (TIE) samples. TDEV can provide information on the noise signal. TDEV values, together with other parameters, are used to evaluate the performance of equipment and systems, often to diagnose a fault which has developed and which impairs customer service.
In order to test the ability of a device to operate in a telecommunication network with noise present, a noise source signal can be injected into the device, which will emulate noise with a recommended characteristic. TDEV can be used as such a characteristic in order to test telecommunications networks. In order to do so, a wander noise signal having a particular frequency profile corresponding to the TDEV needs to be generated. Traditionally, methods of wander noise generation use a Pseudo Random Binary Sequence (PRBS), which produces an approximately white phase spectrum. A sequence generated by a PRBS is then filtered to produce a frequency spectrum that is a satisfactory approximation to the required TDEV wander noise profile.